As a conventional type of tone-in-band transmitter with a distortion reducing circuit based on a direct modulation system, there is, for instance, one in which a modulated wave amplified by a power amplifier is inputted via a directional coupler and an attenuator into a quadrature demodulator, a signal demodulated in the quadrature demodulator is outputted to an amplifier, subtraction (NFB) with a signal to drive the quadrature modulator is executed in an input section of the amplifier, and output from the amplifier is inputted into the quadrature modulator to generate a modulated wave.
FIG. 4 is a block diagram of a main section of a conventional type of tone-in-band transmitter with a distortion reducing circuit based on a direct modulation system. An aural signal introduced from a microphone 401 is amplified by an microphone amplifier 402, subjected to A/D conversion in an A/D convertor 403, and fetched into a DSP (sound processing circuit for modulation) 404.
In the DSP 404, the inputted aural signal is subjected to such a processing required for generation of tone-in-band as band limitation, band division, and frequency shift, and the signal is outputted after being divided to two signals I, Q crossing each other at right angles. The two signals I, Q are subjected to D/A conversion in a D/A convertor 405 and sent to operational amplifiers 406, 407, each for modulation, and each provided in correspondence to the signals I, Q respectively. It should be noted that signals I', Q', demodulated by a quadrature demodulator 415 and amplified in amplifiers 418, 419 or in amplifiers 420, 421 are inputted into input sections of the operational amplifiers 406, 407, each for modulation, respectively, and subtraction (NFB) between the signals I, Q and signals I', Q' is executed.
Then by driving the quadrature modulator 408 with modulated output from the operational amplifiers 406, 407, each for modulation, a modulated wave (tone-in-band) is outputted from the quadrature modulator 408. It should be noted that a signal from a local oscillator 416 is distributed by a power splitter 417 into the quadrature modulator 408. The modulated wave outputted from the quadrature modulator 408 is amplified in a pre-amplifier 409 and an RF power amplifier 410 and then transmitted from an antenna 411. Also, only a progressive wave is detected by a directional coupler 412 from sending power amplified by the RF power amplifier 410, and the progressive wave passes through an attenuator 413 and a phase line 414 each for level adjustment and is sent to a quadrature demodulator 415. It should be noted that a signal (carrier wave) from a local oscillator 416 is distributed by a power splitter 417 to the quadrature demodulator 415 as in the case of the quadrature modulator 408.
In the quadrature demodulator 415, the quadrature relationship between signals is important, and the directional coupler 412 described above is employed to prevent effects by a reflected wave, and also the phase line 414 is provided to accurately align the phase.
Thus in the output from the quadrature demodulator appear the demodulated signals I', Q'. And the signals I', Q' are amplified by the amplifiers 418, 419 or the amplifiers 420, 421 in the next stage respectively and subjected to subtraction in input sections of the operational amplifiers 406, 407 each for modulation, and thus the distortion reducing circuit being operated.
The tone-in-band has tone inserted at a center of the band, but this is a reduced carrier, which is generally adjusted to an output level of the maximum modulation level of around -10 dB. The reduced wave is at first reduced to around -30 dB or below for balance with the quadrature modulator 408, and a level of the carrier is controlled according to DC voltage outputted being superimposed on aural signals of the signals I, Q outputted from the DSP 404.
However in a circuit of the conventional type of tone-in-band transmitter with a distortion reducing circuit based on a direct modulation system, as a balance with the quadrature demodulator 415 is not established only by turning power ON, reduction of a carrier wave may be inappropriate, or because of that the operational amplifier for modulation 406 or 407 may be saturated and disabled, and the operational amplifier 406 or 407 would not run normally.
For this reason, in order to overcome this problem, in a state where functions of the pre-amplifier 409 and the RF power amplifier 410 are stopped with carrier wave inputted only into the quadrature demodulator 415, and DC elements due to imbalance in the quadrature modulator 415 are being outputted as signals I', Q', signals I, Q are sent from the DSP 404, subtraction between the signals I, Q and signals I', Q' is executed in input sections of the operational amplifiers modulations 406, 407, each for modulation. Furthermore, the value of voltage amplified in and outputted from the operational amplifiers 406, 407, each for modulation, is measured, DC voltage values of the signals I, Q at a point of time when the measured value above is 0 V are obtained as DC offset values for the operational amplifiers 406, 407, each for modulation, the DC offset voltage values are stored previously as estimated values in the DSP 404, and operation is started each time operation of the DSP 404 is started according to the estimated values.
Namely, DC voltage values which can be balanced with the quadrature demodulator 415 (DC offset values) are stored as estimated values in the DSP 404 by making use of the phenomenon that the output from the operational amplifiers 406, 407 become 0 instantly when the signals I, Q outputted from the DSP 404 cancel the signals I', Q' due to imbalance in the quadrature demodulator 415, and balance in the quadrature demodulator 415 are always maintained by using the estimated values.
It should be noted that, as this circuit is an NFB circuit, an imbalance element generated in the quadrature modulator 408 appears in the signals I', Q' from the quadrature demodulator 415, and are amplified by the amplifiers 418, 419 or amplifiers 420, 421 and then returned, automatic adjustment being thereby executed.
However, with the conventional technology as described above, although imbalance in a demodulation circuit or in a modulation circuit (quadrature demodulator, quadrature modulator) is eliminated by making use of the estimated values obtained through previous measurement and stored therein, balance in an operational amplifier for modulation and that in a demodulation circuit, a modulation circuit (quadrature demodulator, quadrature modulator) drifts due to such a reason as change in temperature or fluctuation of power. Hence, dispersion is very large, and balance can not appropriately be established only by using the same estimated values each time.
Also, measurement of the estimated value above is not executed for each transmitter, and a DC offset voltage value is measured for a plurality of transmitters each based on the same specifications. An average of the measured values is used as an estimated value for a transmitter having the same specifications. However, actually dispersion due to difference between individual transmitters exists, so that appropriate balance can not always be established.